Generally, when batteries are used in a railway system in order to supply energy during running or to recover regenerative energy at the time of deceleration and halt, many cell batteries are used with being connected to each other in series-parallel combination, because of a large amount of electric energy to be consumed. At this point, it is necessary to obtain an SOC for the running of a vehicle and control of charge/discharge of batteries.
There has been conventionally known a method of estimating an SOC of a battery by a combination of a system using integration of charge and discharge currents and a system using a relation between an open-circuit voltage and the SOC of a secondary battery held in equilibrium (for example, Patent Literature 1 mentioned below). The method disclosed in Patent Literature 1 is to estimate an SOC from current integration at the time of charge or discharge, and to estimate an SOC by estimating the open-circuit voltage during a standby time.
In relation to the open-circuit voltage estimation method, there has been known an example of estimating an open-circuit voltage using a voltage change after charge or discharge (for example, Patent Literature 2 mentioned below).